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Abstract

Here we demonstrate the combination of a semiconductor nanowire and a plasmonic bowtie nanoantenna. A subwavelength InP nanowire was placed precisely in the middle of the nanogap of a gold bowtie nanoantenna with a nanomanipulator installed in a focused ion beam system. We observed a significantly large enhancement (by a factor of 110) of the photoluminescence intensity from this coupled system when the excitation wavelength was at the plasmonic resonance with its polarization parallel to the nanoantenna. Moreover, simulation results revealed that this large enhancement was caused by an interesting interplay between the plasmonic resonance of the nanoantenna and the breakdown of the field suppression effect in the subwavelength nanowire. Our results show that the combination of a nanowire and a nanoantenna gives us a new degree of freedom to design light-matter interactions on a nanoscale.

Figures (6)

Fig. 1 (a) Schematic structure of the sample and coordinate system for numerical calculations. The origin of the coordinate system is at the center of the nanoantenna on the substrate surface. X (Y) is the perpendicular (parallel) direction to the nanoantenna in plane. (b) Nanomanipulation process. We picked up a single nanowire with a diameter of 60 nm and a length of 7 μm and placed it in the nanoantenna gap. (c) SEM image of the fabricated sample. (d) Magnified SEM image of the nanoantenna. The stage was tilted by 45°.

Fig. 2 Wavelength dependence of the electric field enhancement ratio (|Ea|/|E0|)2 calculated by FEM. Ea is the electric field at (X, Y, Z) = (0, 0, d/2) for the structure with the nanoantenna, and E0 is that for the structure without the nanoantenna. d is the nanowire diameter. The gray line shows (|Ea|/|E0|)2 = 1.

Fig. 3 (a)-(c) Distributions of electric field |E| at Z = 25 nm for E// excitation. The electric fields were calculated for structures with (a) only a nanoantenna, (b) only a nanowire, and (c) the nanowire-nanoantenna system. The diameter of the nanowire d is 50 nm here, and the wavelength of the incident light is 660 nm. (d) Cross-sectional plot of |E|2 along the X direction at (Y, Z) = (0, 25 nm) for the structure with the nanowire-nanoantenna system. This cross-sectional plot corresponds to (c). (e) Field distribution at Z = 25 nm for E⊥ excitation. The electric field was calculated for the structure with the nanowire-nanoantenna system. d is 50 nm, and the wavelength of incident light is 660 nm.

Fig. 4 CL intensity mapping images around the nanoantenna measured at room temperature. The emission was filtered with a band-pass filter (the bandwidth was 50 nm). The center wavelengths of the filter were 500 (a), 650 (b), and 800 nm (c). We excited the plasmonic mode with an electron beam with an acceleration voltage of 15 kV and a current of 18 nA. Scale bar, 100 nm.

Fig. 5 (a) Coordinate system for PL measurement, where the antenna size is increased for clarity. x and y are the moving directions of the scanning stage, and we mounted the sample on the scanning stage with y parallel to the nanoantenna. The origin is set at the antenna position. (b) PL spectra measured for E⊥ excitation. The sample temperature was 80 K, the excitation wavelength was 636 nm, and the excitation power was 160 μW. (c)-(f) Mapping images of normalized PL intensity I/IR under various excitation and detection polarization settings: (c) E⊥ (perpendicular to the nanoantenna) excitation and E⊥ detection, (d) E// (parallel to the nanoantenna) excitation and E⊥ detection, (e) E⊥ excitation and E// detection, and (f) E// excitation and E// detection. The PL intensity is normalized by the intensity at a fixed reference position R (IR) in each set of data. The excitation wavelength was 636 nm, and the excitation power was 160 μW. For this measurement, we used a band-pass filter with a bandwidth of 15 nm and a center wavelength of 875 nm to detect the emission from the nanowire. (g)-(j) Mapping images of I/IR with excitation at a wavelength of 532 nm. The polarization of the excitation and the detection in (g)-(j) correspond to (c)-(f), respectively. The excitation power was 80 μW.